----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "topband" <topband@contesting.com>
Sent: Tuesday, May 07, 2013 9:48 AM
Subject: Re: Topband: Problem with compression F connectors on Quad RG-6
CATV installers use almost 100% quad shield in order to keep the signals
inside and not cause interfering leakage (egress); FCC specs are adamant
about that. These specs go back to the 70's. In more recent years the
cable also must keep local RFI (ingress) out.
While they do use tape foil shield cables, CATV systems rarely use quad
shield cables. As a matter of fact I just bought a bunch of drop cable
from a CATV system, and it is all single foil single braid. This is true
for the drop, which has a messenger strand, and the house wiring, which is
identical without a messenger strand.
In the late 70's and early 80's, I was system's engineer at a company that
had dozens of small cable systems. We inherited some systems near an FM/AM
station that had a second harmonic on a local TV channel video frequency.
The former cable system operator had given up, after installing quad
shield and all sorts of special cables. 100% of their problems were
isolated power and CATV grounds, letting the AM signal loop through the
system, and the quad shield developing poor connections letting the FM
harmonic in. We ripped all that stuff out, and went with normal hardline
and good quality drop cable, bonded the cable grounds to the entrance and
breaker panels, and nearly 100% cured the system. We had dumpster load of
special cable that was nothing but connector headaches.
The standard drop cable is a bonded foil with single braid, it has been
that way since the late 1970's. The current best grade is "Brightwire" by
CommScope. Any good cable will far exceed FCC specs without a quad shield.
Single-foil single-shield "Brightwire" has over 120 dB of external shield
current to center conductor current isolation on 160 meters, and it gets
better as you go up in frequency. I can't imagine anyone needing more than
80 dB isolation outside the house, and maybe 100 dB if it is in a noisy
house.
Around here Comcast installers use TFC T-10 quad shield. Ive also seen PPC
trishield used for satellite.
As you can easily see they list their cables by RF noise enviroment from low
for standard dual shield, medium-moderate for premium dual shield, plus high
for trishield and quad for severe with these last 2 also rated for 2 way
use.
http://www.timesfiber.com/TFC_Cable_Book_III.pdf
Go to Page 36
Dont let ANYONE tell you that the shielding type isnt important.
In the 80's I was an engineering manager for Wang Labs broadband networking
R&D department and one of my responsibilities was qualifying all cable
related components for outdoor and indoor use for moderate to severe RF
enviroments which included nuclear carriers and plants, plus oil refineries.
There was a measurable difference between dual and quad shield as well as
percent of braid coverage. These cables plus connectors were tested in the
screen room in the Tempest group.
Tempest was a DOD/CIA Top Secret program tasked to minimize electronic
signal ingress and egress from spies and saboteurs. Since I had maintained
my TS Crypto clearance as a USN Reservist and as an engineering tech and an
engineer for Sanders Associates who was a leader in Tempest technology, I
had full access to the testing.
I have also applied that knowledge at home for ham and consumer products
ever since. It was Tempest starting in the mid 70's that gave me an
understanding of ferrites for RFI, etc and that was applied to Wangs network
as well as at home where I continue to do mortal battle with noise
generators and common mode.
Carl
KM1H
All good topband ops know fine whiskey is a daylight beverage.
_________________
Topband Reflector
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